ORAU Team

Dose Reconstruction Project for NIOSH

Estimation of Neutron Dose Rates from Alpha-Neutron 

Reactions in Uranium and Thorium Compounds 

Document Number: 

ORAUT-OTIB-0024

Effective Date: 04/07/2005

Revision No.: 00

Controlled Copy No.:________

Page 1 of 16

Subject Experts: Robert Hysong, Wesley C. Proctor, Jeffrey J. Vollmer

Document Owner

Approval: Signature on File Date: 03/22/2005

Cindy W. Bloom, Team Leader

Approval: Signature on File Date: 03/22/2005

Judson L. Kenoyer, Task 3 Manager

Concurrence: Signature on File Date: 03/22/2005

Richard E. Toohey, Project Director

Approval: Signature on File Date: 04/07/2005

James W. Neton, Associate Director for Science

Supersedes:

None
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 2 of 16

RECORD OF ISSUE/REVISIONS

ISSUE 

AUTHORIZATION 

DATE

EFFECTIVE 

DATE

REV. 

NO. DESCRIPTION

Draft 12/06/2004 00-A New technical information bulletin to provide 

estimation of neutron doses from uranium and 

thorium compounds that contain low atomic 

number components. Initiated by Cindy W. 

Bloom. 

Draft 01/13/2004 00-B Updated document based on ORAU Team 

comments. Clarified intent of OTIB, added 

additional clarifying information, and a comparison 

of the measured to calculated dose rate. Initiated 

by Robert Hysong.

Draft 03/11/2005 00-C Updated document based on OCAS comments. 

Corrected formatting errors and added generic 

words regarding OTIBs to the purpose. Initiated 

by Cindy W. Bloom.

Draft 03/11/2005 00-D Corrected references to tables. Initiated by 

Robert Hysong.

04/07/2005 04/07/2005 00 First approved issue. Initiated by Cindy W. 

Bloom.
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 3 of 16

1.0

Technical Information Bulletins (TIBs) are general working documents that provide guidance 

concerning the preparation of dose reconstructions at particular sites or categories of sites. TIBs will 

be revised in the event additional relevant information is obtained. TIBs may be used to assist NIOSH 

in the completion of individual dose reconstructions.

PURPOSE

The purpose of this document is to provide a quick estimate of neutron doses from alpha particle 

collisions with low atomic number materials. This document provides an estimate of neutron doses at 

sites that processed thorium and uranium compounds with low atomic number components, but did 

not perform neutron measurements. 

2.0

This TIB estimates neutron production and dose rates from the alpha-neutron reaction in uranium and 

thorium compounds. The chemical forms of uranium considered were UF4 and UF6 as well as the 

uranium oxide forms (UO2, UO3, U3O8) and soda salt (Na2U2O7). Little information could be found 

about the chloride form (UCl4); as a consequence, UCl4 is considered by identifying another target 

material (chemical form) that bounds its neutron emission rate. Based on neutron yield data for 210Po 

and 234U sources, the neutron emission rate of the chloride form would fall below the oxide form (see 

Tables 2-1 and 2-2). Consequently, it is reasonable to assume that the neutron emission rate from 

UCl4 is bounded by UF4. Thorium in the form of a fluoride (ThF4) and a nitrate (Th(NO3)4) were also 

analyzed. Little information could be found on the neutron yield for nitrogen. Based on neutron yield 

data for a 210Po source and a nitrogen target shown in Table 2-2 below, the neutron emission rate 

from the nitrogen form is seven times lower than the oxygen yield. As a consequence, the use of the 

oxygen yield alone for thorium nitrate compounds, Th(NO3)4, will produce a slight overestimate of 

neutron yield. 

SCOPE

Table 2-1. Neutron yields for trace impurities in uranium –234.a

Target

Element

Neutron Yield

per 106 Alphas

(4.7 MeV Alpha)

Neutron Yield

(n/s-Ci)

Neutron Yield

(n/s-g)

Li 1.60E-01 5.92E+03 3.70E+01

Be 4.40E+01 1.63E+06 1.02E+04

B 1.24E+01 4.59E+05 2.87E+03

C 5.10E-02 1.89E+03 1.18E+01

O 4.00E-02 1.48E+03 9.25E+00

F 3.10E+00 1.15E+05 7.17E+02

Na 5.00E-01 1.85E+04 1.16E+02

Mg 4.20E-01 1.55E+04 9.71E+01

Cl 1.00E-02 3.70E+02 2.31E+00

a. Adapted from DOE 2000.
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 4 of 16

Table 2-2. Neutron yields for various elements and a polonium–210 source.a

Target

Element

Neutron Yield

per 106 alphas

(5.3 MeV Alpha)

Neutron Yield 

(n/s-Ci)

Neutron Yield

(n/s-g)

Li 2.60E+00 9.62E+04 4.32E+08

Be 8.00E+01 2.96E+06 1.33E+10

B 2.40E+01 8.88E+05 3.99E+09

C 1.10E-01 4.07E+03 1.83E+07

O 7.00E-02 2.59E+03 1.16E+07

F 1.20E+01 4.44E+05 1.99E+09

Na 1.50E+00 5.55E+04 2.49E+08

Mg 1.40E+00 5.18E+04 2.33E+08

N 1.00E-02 3.70E+02 1.66E+06

Cl 1.10E-01 4.07E+03 1.83E+07

a. Adapted from Auguston and Reilly 1974.

As long as there is an adequate amount of available target material (fluorine, oxygen, sodium, or 

chlorine) and that material is intimately mixed with the source material (uranium or thorium), neutron 

production essentially depends on the amount of the source material present. This TIB analysis made 

the bounding assumption that there is an adequate amount of target material in all the chemical forms 

analyzed which results in the maximum neutron emission rate. 

Neutron dose rates have been calculated for sources of natural uranium, natural thorium, and a 

mixture of thorium isotopes with and without alpha-emitting progeny contribution. Dose rates have 

been calculated for natural uranium (238U, 234U, and 235U) assuming the following: (1) no alphaemitting progeny are present; (2) alpha-emitting progeny are present in secular equilibrium through 

226Ra and 223Ra; and (3) alpha-emitting progeny are present in secular equilibrium through 210Po and 

211Po. Dose rates have been calculated for natural thorium (232Th and 228Th) assuming the presence 

of alpha-emitting progeny in secular equilibrium through 224Ra and assuming the presence of alphaemitting progeny in secular equilibrium through 212Po. Finally, the neutron dose rate for a mixture of 

thorium isotopes containing natural thorium (232Th and 228Th) and 230Th has been evaluated assuming 

the presence of alpha-emitting progeny in secular equilibrium through 224Ra and assuming the 

presence of alpha-emitting progeny in secular equilibrium through 212Po. Radon (222Rn and 223Rn) is 

present in the decay chains of natural uranium (238U and 235U), and 220Rn is present in the natural 

thorium decay chain (232Th). As an inert gas, radon would not be chemically bound with the target 

material; as a consequence, actual neutron yield contribution might be lower from radon and alphaemitting radon progeny than calculated here. The presence or absence of progeny and their degree 

of equilibrium will vary depending on site-specific processes and practices. As a consequence, the 

presence of progeny and degree of equilibrium should be determined on a site-specific basis.
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 5 of 16

3.0

Neutron production increases with the available alpha particle energy as indicated by comparing the 

neutron yields in Table 2-1 (4.7 MeV alpha) with the yields in Table 2-2 (5.3 MeV alpha) for the same 

target element. Likewise, the average and maximum neutron energies resulting from an alpha 

neutron reaction in a target element increase with increasing alpha energy. The resulting neutron 

energies also vary with the angle of neutron emission resulting in a fairly continuous spectrum of 

neutron energies from very low energies to the maximum neutron energy. Table 3-1 presents 

experimental results of the average and maximum neutron energies resulting from 4.0, 4.5, 5.0 and 

5.5 MeV alpha particles bombarding oxygen and fluorine targets. Table 3-2 presents the average and 

maximum neutron energies from several alpha-neutron sources and various target elements. The 

data in Tables 3-1 and 3-2 are in fair agreement; however, the highest alpha energy considered in this 

TIB is 8.78 MeV from 212Po in the 232Th decay chain. Consequently, the average and maximum 

neutron energies shown in Table 3-1 below are expected to increase by a factor determined by linear 

interpolation of the data in Table 3-2 resulting in the average and maximum neutron energies shown 

in Table 3-3. Even though higher energy neutrons can be produced by higher energy alpha particles, 

the fluence per unit dose equivalent is very insensitive to the neutron energies considered in the 232Th 

decay chain (Mlekodaj 2002). The neutron fluence per unit dose equivalent conversion factor varies 

by only 14% in the 1.0- to 10.0-MeV neutron energy range as indicated in Table 3-4 below (Shleien, 

Slaback, and Birky 1998). 

NEUTRON ENERGY SPECTRUM

Table 3-1. Average and maximum (α,n) neutron energies (MeV)

 for various alpha particle energies and oxygen and fluorine targets.a

Source Alpha 

Energy (MeV)

Oxygen Target Element 

Average Maximum 

(MeV)

Fluorine Target Element 

Average Maximum 

(MeV)

4.0 1.71 3.10 0.73 1.80

4.5 1.89 3.60 1.00 2.30

5.0 1.86 4.00 1.12 2.70

5.5 2.02 4.50 1.21 3.20

a. Jacobs and Liskien 1983

Table 3-2. Average and maximum (α,n) neutron energies (MeV) for 238Pu, 239Pu and 241Am alpha particles and oxygen, fluorine and sodium targets.a

Source Alpha 

Energy (MeV)

Oxygen Target 

Average Maximum 

(MeV)

Fluorine Target 

Average Maximum 

(MeV)

Sodium Target 

Average Maximum 

(MeV) 238Pu (5.5 MeV) 2.0 5.8 1.3 3.2 -- --

239Pu (5.2 MeV) 1.9 5.5 1.4 2.8 -- 1.8

241Am (5.48 MeV) -- -- 1.3 2.5 -- --

a. NRC 1991

-- No Data 

Table 3-3. Average and maximum (α,n) neutron energies (MeV)

 for 4.0 to 8.8 MeV alpha sources and oxygen and fluorine targets.a
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 6 of 16

Source Alpha 

Energy (MeV)

Oxygen Target Element 

Average Maximum 

(MeV)

Fluorine Target Element 

Average Maximum 

(MeV)

4.0 1.71 3.10 0.73 1.80

4.5 1.89 3.60 1.00 2.30

5.0 1.86 4.00 1.12 2.70

5.5 2.02 4.50 1.21 3.20

6.0 2.10 4.95 1.41 3.65

6.5 2.19 5.41 1.56 4.11

7.0 2.28 5.90 1.72 4.57

7.5 2.37 6.33 1.87 5.03

8.0 2.46 6.79 2.03 5.49

8.5 2.55 7.25 2.19 5.95

8.8 2.60 7.53 2.28 6.23

a. Based on Scaling Data from Table 2-3 from Jacobs and Liskien 1983

Table 3-4 Mean quality factors, Q, and fluence per

unit dose equivalent for monoenergetic neutrons.

Neutron 

Energy 

(MeV)

Quality 

Factor

(Q)

Fluence per Unit Dose 

Equivalent

(neutron cm-2 rem-1)

Dose Equivalent per 

Unit Fluence

(rem per neutron cm-2)

Dose Equivalent Rate 

per Unit Neutron Flux

(rem h-1 per neutron s-1 cm-2)

2.5E-08 2.0 9.8E+08 1.0E-09 3.7E-06

1.0E-07 2.0 9.8E+08 1.0E-09 3.7E-06

1.0E-06 2.0 8.1E+08 1.2E-09 4.4E-06

1.0E-05 2.0 8.1E+08 1.2E-09 4.4E-06

1.0E-04 2.0 8.4E+08 1.2E-09 4.3E-06

1.0E-03 2.0 9.8E+08 1.0E-09 3.7E-06

1.0E-02 2.5 1.0E+09 9.9E-10 3.6E-06

1.0E-01 7.5 1.7E+08 5.9E-09 2.1E-05

5.0E-01 11 3.9E+07 2.6E-08 9.2E-05

1.0E+00 11 2.7E+07 3.7E-08 1.3E-04

2.5E+00 9 2.9E+07 3.4E-08 1.3E-04

5.0E+00 8 2.3E+07 4.3E-08 1.6E-04
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 7 of 16

Neutron 

Energy 

(MeV)

Quality 

Factor

(Q)

Fluence per Unit Dose 

Equivalent

(neutron cm-2 rem-1)

Dose Equivalent per 

Unit Fluence

(rem per neutron cm-2)

Dose Equivalent Rate 

per Unit Neutron Flux

(rem h-1 per neutron s-1 cm-2)

7.0E+00 7 2.4E+07 4.2E-08 1.5E-04

1.0E+01 6.5 2.4E+07 4.2E-08 1.5E-04

1.4E+01 7.5 1.7E+07 5.9E-08 2.1E-04

2.0E+01 8 1.6E+07 6.3E-08 2.3E-04

 a. Adapted from Shleien, Slaback, and B. K. Birky 1998 and NCRP 1971. 

The following assumptions were made to convert from neutron production rate per gram of source 

material to neutron dose rate at 1- and 3-ft distances. 

1. Point source geometry has been assumed to calculate the neutron flux at distances of 1 and 3 

feet from the source.

2. The average neutron energy from alpha-neutron reactions is 2.0 MeV; consequently a dose 

equivalent rate –to– fluence rate conversion factor for 2.0 MeV neutrons of 1.3 × 10-4 rem/hr 

per neutron/cm2

-s (Shleien, Slaback, and Birky 1998) was used to determine dose rates. Selfshielding (attenuation and scatter) within the source-target compound matrix is assumed to be 

negligible. This is a very conservative assumption, because containers exceeding a few 

kilograms of material will result in a lower neutron energy spectrum and lower neutron flux 

outside the container due to neutron scatter and attenuation within the source. 

3. Dose rate calculations are based on a source of one gram of natural uranium or thorium 

(source material). The entire mass of uranium compounds can be assumed conservatively to 

be attributable to the uranium present in the compounds considered here. Site-specific 

adjustments for mass might be necessary for some thorium compounds because the mass of 

thorium in a solution of thorium nitrate (Th (NO3)4 ) represents only 48% of a gram of Th (NO3)4

and 75% of a gram of ThF4. Similarly, the mass of uranium in a solution of uranyl nitrate 

[UO2(NO3)2] represents only 60% of the total mass of the solution. 

A Microsoft® Excel spreadsheet, Alpha-NforTIB12-01-04Draft2.xls, was created to perform the 

calculations in this TIB. The spreadsheet is available to calculate neutron production rates and doses 

from modified source compositions and is useful for inputting known masses of material to obtain an 

estimate of neutron dose rate. 

4.0

Only a limited amount of thick target (α,n) yield data is available in the literature. Table 4-1 below 

presents neutron yield data from Table 6-5 of DOE Standard –1136, 2000, Salmon and Hermann, 

1992, and Auguston and Reilly, 1974. The neutron yield data presented in DOE 2000 were originally 

published in Table 11-3 of Passive Nondestructive Assay of Nuclear Materials, NUREG/CR-5550 

(NRC 1991). The neutron yields in the DOE 2000 are based on pure oxides and fluorides and do not 

account for either the neutron contribution from spontaneous fission neutrons from 238U, 235U and 234U, 

or the presence of other low-atomic-number impurities (DOE 2000). The ratio of neutron yield data for 

oxygen in Salmon and Hermann (1992) to the yield data reported by DOE (2000) ranges from 2.8 to 

2.9. The ratio of neutron yield data for oxygen in the Auguston and Reilly (1974) to the yield data in 

Salmon and Hermann (1992) are in good agreement ranging from 0.9 to 0.93. 

NEUTRON YIELDS
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 8 of 16

The ratio of neutron yield data for fluorine in Salmon and Hermann (1992) to the yield data in DOE 

(2000) ranges from 0.6 to 1.9. The ratio of the neutron yield data for fluorine in Salmon and Hermann 

(1992) to the yield data in Augustin and Reilly (1974) are in better agreement ranging from 0.4 to 1.1. 

Table 4-2 below presents neutron yield data for a 234U source and several target elements from Table 

6-6 of DOE 2000, Salmon and Hermann 1992, and Auguston and Reilly 1974. The yield data from 

DOE 2000, Table 6-6 are in very good agreement with the Salmon and Hermann (1992) and 

Auguston and Reilly (1974) data. The yields from Salmon and Hermann (1992) are greater than the 

DOE 2000, Table 6-6 data by a factor of only 0.89 to 1.11. 

The neutron yield data from Salmon and Hermann is used in the ALPHN computer code to calculate 

the neutron production rate in canisters of vitrified high level waste. Some degree of code verification 

was obtained from the ORIGEN-S code on the same example problem. The difference in results was 

0.5% (Salmon and Hermann 1992). Validation of the ORIGEN-S code by comparing its results to 

the results of experiments conducted at Savannah River Laboratory has also been performed. The 

experiments used glass composition containing a uniformly distributed known amount of 238Pu. The 

total (,n) neutron source predicted by ORIGEN-S was 2% greater than the average of three 

samples measured at Savannah River Laboratory. These results can be taken as some degree of 

validation of ALPHN (Salmon and Hermann 1992). Consequently, the neutron yield data from 

Salmon and Hermann (1992) has been chosen to calculate neutron dose rates in this TIB. In 

addition, the Salmon and Hermann (1992) yield data appears to be in good agreement with Table 6-6 

of DOE 2000, and with Auguston and Reilly 1974. Lastly, the Salmon and Hermann (1992) reference 

provides a method to calculate the neutron yields for all of the decay progeny in this TIB based on the 

average alpha energy of the source radionuclide.

Table 4-1. Neutron yields in neutrons per second per gram of source isotope (n/s-g)a

.

Source 

Isotope/Ave. 

Alpha Energy 

(MeV)

DOE Standard

Table 6-5

Target

Salmon and Hermann

Table 3 Interpolation

Target

Auguston and Reilly

Tables 7.1 and 7.2

Target

Oxygen Fluoride Oxygen Fluoride Oxygen Fluoride

234U 4.77 3.00E+00 5.80E+02 8.62E+00 7.81E+02 9.15E+00 1.56E+03

238U 4.19 8.30E-05 2.80E-02 2.42E-04 1.71E-02 2.64E-04 4.50E-02

235U 4.40 7.10E-04 8.00E-02 1.97E-03 1.48E-01 2.11E-03 3.60E-01

232Th 4.00 2.20E-05 -- 5.68E-05 3.57E-03 -- --

a. DOE 2000, Salmon and Hermann 1992, Auguston and Reilly 1974

-- No data

Table 4-2 Neutron yields (n/s-g) for 234U with trace Impurities a

.

Target Element DOE Standard 

Table 6-5

DOE Standard 

Table 6-6

Salmon & Hermann 

Table 3 Interpolation

Auguston & Reilly 

Tables 7.1 & 7.2

Fluoride 5.80E+02 7.17E+02 7.81E+02 1.56E+03

Oxide 3.00E+00 9.25E+00 8.26E+00 9.15E+00
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 9 of 16

Target Element DOE Standard 

Table 6-5

DOE Standard 

Table 6-6

Salmon & Hermann 

Table 3 Interpolation

Auguston & Reilly 

Tables 7.1 & 7.2

Sodium -- 1.16E+02 1.29E+02 1.98E+02

a. DOE 2000, Salmon and Hermann 1992, Auguston and Reilly 1974

-- No Data

Tables 4-3 through 4-6 list neutron yields for fluorine, oxygen, and sodium targets for sources of 238U, 

235U, 232Th, 230Th, and associated progeny. Soda salt (Na2U2O7) yields have been calculated for 

uranium isotopes on an atom percent weighted basis (22.22% Na and 77.77% O). Target yields have 

been derived based on the data and methodology presented in Salmon and Hermann (1992).

The neutron yields for fluoride, sodium, oxygen, and soda salt listed in Tables 4-2 through 4-5 were 

used to derive a neutron multiplication factor. This factor accounts for the production of neutrons from 

alpha-emitting progeny in the 235U, 238U and 232Th decay chains. For example, the neutron 

multiplication factor for the 238U decay chain has been calculated by dividing the sum of the alpha 

abundance weighted neutron yields through 226Ra by the yield for 238U plus 234U. The neutron 

multiplication factors for the 235U and 232Th chains were calculated in the same manner. The resulting 

neutron multiplication factors are listed in Tables 4-7 and 4-8 below. 

To account for the radioactivity distribution of isotopes in natural uranium, activity-weighted 

multiplication factors (see Tables 4-3 and 4-4) have been calculated as the product of the neutron 

multiplication factor and the fraction of alpha-emitting activity in each chain. For natural uranium in 

equilibrium through 226Ra and 223Ra, the 238U chain accounts for 95.1% of the alpha-emitting activity 

while 235U accounts for 4.85% of the alpha-emitting activity. For natural uranium in equilibrium 

through 210Po and 211Po, the 238U chain accounts for 97% of the alpha-emitting activity while 235U 

accounts for 3% of the alpha-emitting activity.

Table 4-3. Neutron yields for 238U chain isotopes.a

Isotope

Average 

alpha energy 

(MeV)

Alpha 

emission % 

abundance

Alpha 

abundance 

weighted O 

yield (n/s-Ci)

Alpha 

abundance 

weighted Na 

yield (n/s-Ci)

Alpha 

abundance 

weighted soda 

salt yield (n/s-Ci)

Alpha 

abundance 

weighted F yield 

(n/s-Ci)

U-238 4.194 1.00E+02 7.19E+02 6.56E+03 2.02E+03 5.09E+04

U-234 4.773 1.00E+02 1.38E+03 2.07E+04 5.68E+03 1.25E+05

Th-230 4.665 1.00E+02 1.24E+03 1.71E+04 4.77E+03 1.07E+05

Ra-226 4.774 1.00E+02 1.38E+03 2.07E+04 5.69E+03 1.25E+05

Rn-222 5.489 1.00E+02 2.48E+03 5.49E+04 1.41E+04 2.84E+05

Po-218 6.001 1.00E+02 3.35E+03 9.43E+04 2.35E+04 4.54E+05

At-218 6.693 1.00E+02 5.00E+03 1.62E+03 4.25E+03 7.63E+03

Bi-214 5.461 2.10E-02 5.11E-01 1.12E+01 2.89E+00 5.82E+01

Po-214 7.686 1.00E+02 7.86E+03 2.59E+05 6.36E+04 1.35E+06

Pb-210 3.720 1.90E-06 6.31E-06 3.62E-05 1.29E-05 3.94E-04

Bi-210 4.625 1.30E-04 1.55E-03 2.05E-02 5.76E-03 1.31E-01

Po-210 5.304 1.00E+02 2.17E+03 4.48E+04 1.17E+04 2.38E+05

Total U-238 to Ra-226 4.72E+03 6.51E+04 1.81E+04 4.08E+05

Activity-weighted neutron multiplication 

factor U-238 to Ra-226

2.14E+00 2.27E+00 2.24E+00 2.21E+00

Total U-238 to Po-210 2.56E+04 5.19E+05 1.35E+05 2.74E+06

Activity-weighted neutron multiplication 

factor U-238 to Po-210

1.18E+01 1.85E+01 1.71E+01 1.51E+01

a. Adapted from Salmon and Hermann (1992).
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 10 of 16

Table 4-4. Neutron yields for 235U chain isotopes.a

Isotope

Average 

alpha energy 

(MeV)

Alpha 

emission % 

abundance

Alpha 

abundance 

weighted O 

yield (n/s-Ci)

Alpha 

abundance 

weighted Na 

yield (n/s-Ci)

Alpha 

abundance 

weighted soda 

salt yield (n/s-Ci)

Alpha 

abundance 

weighted F yield 

(n/s-Ci)

U-235 4.378 1.00E+02 9.10E+02 9.57E+03 2.83E+03 6.83E+04

Pa-231 4.923 1.00E+02 1.57E+03 3.20E+04 8.33E+03 1.50E+05

Ac-227 4.931 1.38E+00 2.18E+01 3.59E+02 9.67E+01 2.09E+03

Th-227 5.902 1.00E+02 3.18E+03 8.66E+04 2.17E+04 4.21E+05

Ra-223 5.697 1.00E+02 2.83E+03 7.07E+04 1.79E+04 3.52E+05

Rn-219 6.812 1.00E+02 5.34E+03 1.73E+05 4.27E+04 8.23E+05

Po-215 7.386 1.00E+02 7.00E+03 2.29E+05 5.64E+04 1.15E+06

At-215 8.026 1.00E+02 8.86E+03 2.92E+05 7.17E+04 1.58E+06

Bi-211 6.550 1.00E+02 4.58E+03 1.48E+05 3.64E+04 6.90E+05

Po-211 7.420 9.89E+01 7.02E+03 2.30E+05 5.66E+04 1.16E+06

Total U-235 to Ra-223 8.51E+03 1.99E+05 5.09E+04 9.94E+05

Alpha activity-weighted neutron 

multiplication factor U-235 to Ra-223

4.54E-01 1.01E+00 8.71E-01 7.05E-01

Total U-235 to Po-211 4.13E+04 1.27E+06 3.15E+05 6.39E+06

Alpha activity-weighted neutron 

multiplication factor U-235 to Po-211

1.36E+00 3.99E+00 3.33E+00 2.81E+00

a. Adapted from Salmon and Hermann (1992).

Table 4-5. Neutron yields for 232Th chain isotopes.a

Isotope

Average 

alpha energy 

(MeV)

Alpha 

emission % 

abundance

Alpha 

abundance 

weighted O 

yield (n/s-Ci)

Alpha 

abundance 

weighted Na 

yield (n/s-Ci)

Alpha 

abundance 

weighted soda 

salt yield (n/s-Ci)

Alpha 

abundance 

weighted F yield 

(n/s-Ci)

Th-232 4.000 1.00E+02 5.18E+02 3.39E+03 1.16E+03 3.25E+04

Th-228 5.399 1.00E+02 2.33E+03 5.00E+04 1.29E+04 2.62E+05

Ra-224 5.675 1.00E+02 2.79E+03 6.90E+04 1.75E+04 3.45E+05

Rn-220 6.288 1.00E+02 3.98E+03 1.22E+05 3.03E+04 5.75E+05

Po-216 6.779 1.00E+02 5.25E+03 1.70E+05 4.19E+04 8.06E+05

Bi-212 6.051 3.59E+01 1.24E+03 5.31E+04 1.28E+04 2.48E+05

Po-212 8.784 1.00E+02 7.41E+03 2.34E+05 5.79E+04 1.34E+06

Total Th-232 to Ra-224 5.64E+03 N/A N/A 6.39E+05

Neutron multiplication factor Th-232 to 

Ra-224

1.09E+01 N/A N/A 1.97E+01

Total Th-232 to Po-212 2.35E+04 3.61E+06

Neutron multiplication factor Th-232 to 

Po-212

4.54E+01 1.11E+02

a. Adapted from Salmon and Hermann (1992).

Table 4-6. Neutron yields for 230Th.a

Isotope

Average 

alpha energy 

(MeV)

Alpha 

emission % 

abundance

Alpha 

abundance 

weighted O 

yield (n/s-Ci)

Alpha 

abundance 

weighted Na 

yield (n/s-Ci)

Alpha 

abundance 

weighted soda 

salt yield (n/s-Ci)

Alpha 

abundance 

weighted F yield 

(n/s-Ci)

Th-230 4.665 1.00E+02 1.24E+03 1.71E+04 4.77E+03 1.07E+05

Neutron multiplication factor 1.00E+00 N/A N/A 1.00E+00

a. Adapted from Salmon and Hermann (1992).

Table 4-7. Multiplication factors for decay chains 

in equilibrium with 223Ra, 226Ra, and 224Ra.

Low Z element U-235 U-238 Th-232

Oxygen 9.35 2.25 10.9

Soda salt (Na2O7) 18.0 2.36 N/A

Fluorine 14.5 2.32 19.7
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 11 of 16

Table 4-8. Multiplication factors for decay chains 

in equilibrium with 211Po, 210Po, and 212Po.

Low Z element U-235 U-238 Th-232

Oxygen 45.1 12.2 45.4

Soda salt (Na2O7) 111.0 17.6 N/A

Fluorine 93.6 15.6 111.0

5.0

Following the chemical separation of uranium from other elements, naturally occurring uranium 

consists of 238U, 235U, and 234U. Table 5-1 summarizes natural uranium properties and isotopic 

composition. The neutron flux from a gram of natural uranium (0.99284 g 238U, 0.000053 g 234U, and 

0.00711 g 235U) following chemical separation is calculated as the sum of the products of each isotope 

fraction per gram, isotope specific activity, and isotope neutron yield. The neutron flux for each 

chemical compound of interest [UO2, UO3, U3O8, soda salt (Na2U2O7), UF4, and UF6] has been 

calculated. The neutron flux at 1 and 3 ft. was calculated as the quotient of the neutron production 

rate per gram of natural uranium and the surface area of a sphere with a radius of 1 and 3 ft (surface 

areas of 1.167 × 104 cm2 and 1.051 × 105 cm2

, respectively). The neutron flux was converted to dose 

rate using a conversion factor of 1.3 × 10-4 rem/hr per neutron/cm2

-s. The neutron dose rates at 1 and 

3 ft from 1 gram of natural uranium with no alpha-emitting progeny present are listed in Table 5-2 

below. 

URANIUM COMPOUND DOSE RATES

Table 5-1. Natural uranium isotopes and properties.

Property U-234 U-235 U-238

Weight percent 5.30E-03 7.11E-01 9.9284E+01

Activity percent 4.89E+01 2.2E+00 4.89E+01

Half-life (Y) 2.45E+05 7.04E+8 4.47E+09

Specific activity (Ci/g) 6.25E-03 2.16E-6 3.36E-07

Table 5-2. Natural uranium-per-gram dose rates 

without alpha-emitting progeny.a

Chemical form

Dose rate (no progeny) rem/hr-gram

1 ft 3 ft

UO2/UO3/

U3O8 (ore)

7.91E-12 8.79E-13

Na2U2O7 (soda salt) 2.89E-11 3.21E-12

UF4/UF6 6.62E-10 7.36E-11

 a. Uranium isotopes following chemical separation 

According to DOE (2000), neutrons of approximately 2 MeV energy are generated by the interaction 

of alpha particles from uranium with the nuclei of fluoride and other low atomic number atoms. The 

magnitude of neutron flux will vary based on the total activity of uranium (which is a function of 

enrichment) and the chemical compound. In the case of UF6, the typically measured neutron dose 

rates for large cooled storage cylinders are as follows:

Natural-5% enrichment: 0.01-0.2 mrem/h (no distance specified in DOE 2000)

Very high enrichment (97+%): 2-4 mrem/h (contact) and 1-2 mrem/h (3 ft) 

Assuming that a “large” storage container is 100 pounds and assuming that no progeny are present, 

the Microsoft® Excel spreadsheet, Alpha-NforTIB12-01-04Draft2.xls, created to perform the 

calculations in this TIB yields the following results:
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 12 of 16

Natural-5% enrichment: 0.03-0.14 mrem/h (1 ft) and 0.003-0.016 mrem/h (3 ft) 

Very high enrichment (97+%): 4.0 mrem/h (1 ft) and 0.45 mrem/h (3 ft)

The neutron dose rates calculated by the Microsoft® Excel spreadsheet for the case of natural-5% 

enrichment are in the same range as those in DOE 2000. For the very HEU case, neutron dose rates 

calculated by the Microsoft® Excel spreadsheet are underestimated by a factor of 2 – 4 at 3 ft due to 

the point source geometry assumed. 

For calculation of the neutron dose rates from uranium with progeny present, an effective neutron 

multiplication factor for natural uranium in secular equilibrium with radium and polonium is calculated 

for each compound as the sum of the activity-weighted multiplication factors (see Tables 4-3 and 4-4) 

for 238U and 235U. The effective neutron multiplication factors are presented in Tables 5-3 and 5-4 

below. When alpha-emitting progeny are present in natural uranium, the effective neutron 

multiplication factors are multiplied by the neutron dose rate resulting from natural uranium (238U, 234U, 

235U) when no other alpha-emitting progeny are present.

Table 5-3. Effective neutron multiplication 

factors for natural uranium in equilibrium 

with radium.

Low Z element Natural uranium

Oxygen 2.59

Soda salt (Na2O7) 3.11

Fluorine 2.91

Table 5-4. Effective neutron multiplication 

factors for natural uranium in equilibrium 

with polonium.

Low Z element Natural uranium

Oxygen 13.2

Soda salt (Na2O7) 20.4

Fluorine 17.9

Table 5-5 lists neutron dose rates from naturally occurring uranium assuming the presence of alphaemitting progeny in secular equilibrium through 226Ra in the 238U chain and through 223Ra in the 235U 

chain. Table 5-5 dose rates are calculated as the product of the dose rates listed in Table 5-2 and the 

effective neutron multiplication factors listed in Table 5-3. The presence or absence of progeny and 

their degree of equilibrium will vary depending on site-specific processes and practices. As a 

consequence, the presence of progeny and degree of equilibrium should be determined on a sitespecific basis. 

Table 5-5. Natural uranium-per-gram dose 

rates with alpha-emitting progeny in secular 

equilibrium with 226Ra in the 238U chain and 

223Ra in the 235U chain.

Chemical form

Dose rate rem/h-gram

1 ft 3 ft

UO2/UO3/

U3O8 (ore)

2.05E-11 2.28E-12

Na2U2O7 (soda salt) 9.00E-11 1.00E-11

UF4/UF6 1.93E-09 2.14E-10
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 13 of 16

Table 5-6 lists neutron dose rates from naturally occurring uranium assuming the presence of alphaemitting progeny in secular equilibrium through 210Po in the 238U chain and through 211Po in the 235U 

chain. Table 5-6 dose rates are calculated as the product of the dose rates listed in Table 5-2 and the 

effective neutron multiplication factors listed in Table 5-4. 

Table 5-6. Natural uranium-per-gram dose 

rates with alpha-emitting progeny in secular 

equilibrium with 210Po in the 238U chain and 

211Po in the 235U chain

Chemical form

Dose rate rem/h-gram

1 ft. 3 ft. 

UO2/UO3/

U3O8 (ore)

1.04E-10 1.16E-11

Na2U2O7 (soda salt) 5.90E-10 6.55E-11

UF4/ UF6 1.19E-08 1.32E-09

6.0

Following the chemical separation of thorium from other elements in ore, naturally occurring thorium 

consists of 232Th and 228Th in secular equilibrium. Within 25 days, 224Ra is in secular equilibrium with 

the 232Th and 228Th; as a consequence, neutron dose rates have been calculated for natural thorium 

compounds assuming the presence of alpha-emitting progeny in secular equilibrium through 224Ra. In 

addition, neutron dose rates from the thorium alpha-n reaction have been calculated assuming the 

presence of alpha-emitting progeny in secular equilibrium through 212Po. As noted for uranium, the 

presence or absence of progeny and their degree of equilibrium will vary depending on site-specific 

processes and practices. As a consequence, the presence of progeny and degree of equilibrium 

should be determined on a site-specific basis. 

THORIUM COMPOUND DOSE RATES

Table 6-1. Natural thorium isotopes and 

properties.

Property Th-232 Th-228

Weight percent ~100 1.34E-08

Activity percent 5.00E+01 5.00E+01

Half-life (Y) 1.41E+10 1.91E+00

Specific activity (Ci/g) 1.10E-07 8.20E+02

The neutron flux from a gram of natural thorium (~1.0 g 232Th and 1.34 × 10-10 g 228Th) is calculated as 

the product of the 232Th mass fraction per gram, 232Th specific activity, 232Th neutron yield (in the 

compound of interest) and the neutron multiplication factor. The neutron multiplication factors for 

232Th in secular equilibrium through 224Ra and through 212Po are listed in Table 4-5. The neutron flux 

for each chemical compound of interest [Th (NO3)4 and ThF4] has been calculated. The neutron flux at 

1 and 3 ft. is calculated as the quotient of the neutron production rate per gram of natural thorium and 

the surface area of a sphere with a radius of 1 and 3 ft (surface areas of 1.167 × 104 cm2 and 1.051 

× 105 cm2

, respectively). The neutron flux was converted to dose rate using a conversion factor of 1.3 

× 10-3 rem/hr per neutron/cm2

-s. Neutron dose rates at 1 and 3 ft from 1 g of natural thorium in 

secular equilibrium through 224Ra and through 212Po are listed in Tables 6-2 and 6-3, respectively.
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 14 of 16

Table 6-2. Natural thorium per gram 

dose rates (secular equilibrium through 

224Ra).

Chemical form

Dose rate rem/hr-gram

1 ft 3 ft

Th(NO3)4 6.89E-12 7.65E-13

ThF4 7.81E-10 8.68E-11

Table 6-3. Natural thorium per gram 

dose rates (secular equilibrium through 

212Po).

Chemical form

Dose rate rem/hr-gram

1 ft 3 ft

Th(NO3)4 2.87E-11 3.19E-12

ThF4 4.41E-09 4.90E-10

Thorium bearing ores also contain varying amounts of uranium, and the thorium decay progeny of the 

naturally occurring isotopes of uranium would also be present in chemically separated thorium. The 

most important of the thorium isotopes and decay progeny in chemically separated thorium 

compounds are 232Th, 228Th, 224Ra, 230Th, and 227Th. The half-life of 227Th is only 18.7 days, and its 

223Ra progeny has a half-life of only 11.4 days. Following purification of the thorium from ore, the 

227Th and 223Ra contribution to neutron dose will decrease rapidly. In addition, the activity ratio of 

230Th to 227Th would probably follow the ratio of 238U to 235U in natural uranium (22.2:1.0 on an activity 

basis). As a consequence, even after initial purification of thorium, the neutron flux component from 

227Th would be negligible in comparison to the 230Th neutron flux contribution. As a result, 227Th and 

223Ra flux contributions have been neglected.

It is reasonable and conservative to assume secular equilibrium between 232Th, 228Th, and 224Ra in 

calculating the alpha-neutron dose rate, but the activity of 226Ra is not in secular equilibrium with 230Th 

until approximately 7,000 years. As a consequence, the contribution to neutron flux from 226Ra can be 

neglected unless site-specific data or processes indicate that 226Ra would be present. 

The neutron flux for a gram mixture containing thorium isotopes (232Th in equilibrium with 228Th and 

224Ra and 230Th with no progeny) is calculated as the sum of the 232Th neutron flux contribution 

discussed above and the 230Th contribution. The mass fraction of 232Th and 230Th will need to be 

determined on a site-specific basis. This analysis is based on the thorium isotope mixture identified at 

the Mallinckrodt site, which contained 88.4% 232Th and 11.6% 230Th by weight. The neutron flux is 

calculated as the sum of the products of the 232Th mass fraction per gram, 232Th specific activity, 232Th 

neutron yield, and 232Th neutron multiplication factor and the product of the 230Th mass fraction, 230Th 

specific activity, and 230Th neutron yield.

The neutron flux at 1 and 3 ft. was calculated as the quotient of the neutron production rate per gram 

of natural uranium and the surface area of a sphere with a radius of 1 and 3 ft (surface areas of 1.167 

× 104 cm2 and 1.051 × 105 cm2

, respectively). The neutron flux was converted to dose rate using a 

conversion factor of 1.3 × 10-3 rem/hr per neutron/cm2

-s. Neutron dose rates at 1 and 3 ft from 1 g of 

thorium isotope mix containing 232Th in secular equilibrium through 224Ra and through 212Po are listed 

in Tables 6-4 and 6-5, respectively.
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 15 of 16

Table 6-4. Thorium isotope mix per 

gram dose rates (232Th in secular 

equilibrium through 224Ra).

Chemical form

Dose rate rem/hr-gram

1 ft 3 ft

Th(NO3)4 3.24E-08 3.60E-09

ThF4 2.80E-06 3.11E-07

Table 6-5. Thorium isotope mix per 

gram dose rates (232Th in secular 

equilibrium through 212Po).

Chemical 

form

Dose rate rem/hr-gram

1 ft 3 ft

Th(NO3)4 3.25E-08 3.61E-09

ThF4 2.80E-06 3.11E-07

The analysis of neutron production by the alpha-neutron reaction identified the forms of uranium and 

thorium that would produce neutrons at the highest rates as UF4, UF6, and ThF4. In addition, the 

analysis considered the uranium oxide forms and soda salt (Na2U2O7) as well as thorium nitrate 

[Th(NO3)4]. Little information could be found about UCl4, so the dose from UCl4 was bounded by 

identifying another form (UF4), which results in a larger dose than the UCl4 form.
Effective Date: 04/07/2005 Revision No. 00 Document No. ORAUT-OTIB-0024 Page 16 of 16

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